The website of Britain's National Health Service notifies users of online problems caused by a global cyberattack that originated in Ukraine earlier this year. The attack hit more than 65 countries, raising new concerns about whether the United States should act more aggressively against cyberattacks that could change election tallies or disable power grids and key military infrastructure. (Cover: AFP/Getty Images/Daniel Leal-Olivas)

The next major conflict between world powers may not begin at sea or along a disputed border, but in cyberspace. In the past decade, hackers have targeted voting systems in the United States, electrical grids in Ukraine, uranium enrichment facilities in Iran and hospitals, universities and major corporations around the world. The attacks have focused new attention on whether the United States is acting quickly enough to protect computer networks serving critical infrastructure, from military bases to power plants. Cybersecurity experts say companies holding sensitive data are particularly vulnerable to digital attacks, such as the recent hack of the Equifax credit reporting agency that potentially affects 145.5 million U.S. consumers. The United Nations is working to develop international rules for cyberwarfare, but the effort faces major hurdles, including deciding how even to define a cyberweapon. Allegations that Russia used social media to disrupt last year's presidential election are another focus of concern as the United States prepares for the 2018 congressional elections.